1. Field of the Invention
The present invention relates to a magnetic bearing device and a turbo molecular pump with the magnetic bearing device mounted thereto. More specifically, the present invention relates to a magnetic bearing device capable of lowering the costs required for the manufacture, installation, or the like of a turbo molecular pump by reducing the number of elements of an amplifier circuit that drives, through excitation, electromagnets as well as the number of wires of a cable that connects the each electromagnet and the amplifier circuit to each other, and to a turbo molecular pump with the magnetic bearing device mounted thereto.
2. Description of the Related Art
With the development of electronics in recent years, demands for semiconductors for forming memories, integrated circuits, etc. are rapidly increasing.
Those semiconductors are manufactured such that impurities are doped into a semiconductor substrate with a very high purity to impart electrical properties thereto, or semiconductor substrates with minute circuit patterns formed thereon are laminated.
Those manufacturing steps must be performed in a chamber with a high vacuum state so as to avoid influences of dust etc. in the air. This chamber is generally evacuated by a vacuum pump. In particular, a turbo molecular pump is widely used since it entails little residual gas and is easy of maintenance.
The semiconductor manufacturing process includes a number of steps in which various process gases are caused to act onto a semiconductor substrate, and the turbo molecular pump is used not only to evacuate the chamber but also to discharge those process gases from the chamber.
Further, in equipment for an electron microscope etc., a turbo molecular pump is used to create a high vacuum state in the chamber of the electron microscope etc. in order to prevent refraction etc. of an electron beam caused by the presence of dust or the like.
Such a turbo molecular pump is composed of a turbo molecular pump main body for evacuating the chamber of a semiconductor manufacturing apparatus, an electron microscope, or the like, and a control device for controlling the turbo molecular pump main body.
FIG. 14 is a vertical sectional view of the turbo molecular pump main body.
In FIG. 14, a turbo molecular pump main body 100 includes an outer cylinder 127 with an intake hole 101 formed on top thereof. Provided inside the outer cylinder 127 is a rotor 103 having in its periphery a plurality of rotor blades 102a, 102b, 102c, . . . serving as turbine blades for sucking and discharging gas and formed radially in a number of stages.
At the center of the rotor 103, a rotor shaft 113 is mounted with being supported in a levitating state in the air and controlled in position, for example, by a 5-axis control magnetic bearing.
Upper radial electromagnet 104 includes four electromagnets arranged in pairs in X- and Y-axis and plus- and minus-side directions (although not shown in the drawing, those electromagnets are referred to as 104X−, 104Y+, and 104Y−, as necessary). Further, there is provided an upper radial sensor 107 constituted of four electromagnets arranged in close proximity to and in correspondence with the upper radial electromagnets 104. The upper radial sensor 107 detects radial displacement of the rotor 103, transmitting a detection signal to a control device 200 (shown in FIG. 15).
In this control device 200, the upper radial electromagnets 104 are excited and controlled by a magnetic bearing control circuit 201 having a PID adjusting function, on the basis of a displacement signal detected by the upper radial sensor 107, thus the radial position of the upper portion of the rotor shaft 113 being adjusted.
The rotor shaft 113 is formed of a high-magnetic-permeability material (e.g., iron) and is adapted to be attracted by the magnetic force of the upper radial electromagnets 104. Such adjustment is conducted independently in the X-axis direction and the Y-axis direction.
Further, lower radial electromagnets 105 and a lower radial sensor 108 are arranged in the same way as the upper radial electromagnets 104 and the upper radial sensor 107. Like the radial position of the upper portion of the rotor shaft 113, the radial position of the lower portion of the rotor shaft 113 is adjusted (the lower radial electromagnets 105 are similarly referred to as 105X+, 105X−, 105Y+, and 105Y−, as necessary).
Further, axial electromagnets 106A and 106B each are arranged on the upper and lower sides of a metal disc 111 provided in the lower portion of the rotor shaft 113. The metal disc 111 is formed of a high-magnetic-permeability material such as iron. To detect axial displacement of the rotor shaft 113, an axial sensor 109 is provided, which transmits an axial displacement signal to the control device 200.
The axial electromagnets 106A and 106B are excited and controlled by the magnetic bearing control circuit 201 having a PID adjusting function on the basis of the axial displacement signal. The axial electromagnet 106A upwardly attracts the magnetic disc 111 by the magnetic force, and the axial electromagnet 106B downwardly attracts the magnetic disc 111.
In this way, the control device 200 has a function to appropriately control the magnetic force exerted on the metal disc 111 by the axial electromagnets 106A and 106B to magnetically levitate the rotor shaft 113 in the axial direction, thereby retaining the rotor shaft 113 in the space in a non-contact state.
Note that descriptions will be given later on the magnetic bearing control circuit 201.
A motor 121 is equipped with a plurality of magnetic poles, which are arranged circumferentially to surround the rotor shaft 113. The magnetic poles are controlled by the control device 200 to rotate the rotor shaft 113 through an electromagnetic force acting between the rotor shaft 113 and the magnetic poles.
The motor 121 also has an RPM sensor (not shown in the drawing) incorporated to output a detection signal, which is used for detection of RPM of the rotor shaft 113.
A phase sensor (not shown in the drawing) is attached, for example, in the vicinity of the lower radial sensor 108 to detect the phase of rotation of the rotor shaft 113. From detection signals of the phase sensor and the RPM sensor both, the control device 200 detects positions of the magnetic poles.
A plurality of stationary blades 123a, 123b, 123c, . . . are arranged so as to be spaced apart from the rotor blades 102a, 102b, 102c, . . . by small gaps. To downwardly transfer the molecules of exhaust gas through collision, the rotor blades 102a, 102b, 102c, . . . are inclined by a predetermined angle with respect to a plane perpendicular to the axis of the rotor shaft 113.
Similarly, the stationary blades 123 are also inclined by a predetermined angle with respect to a plane perpendicular to the axis of the rotor shaft 113, and extend toward the inner side of the outer cylinder 127 to be arranged alternately with the rotor blades 102.
The stationary blades 123 are supported at one end by being inserted into gaps between a plurality of stationary blade spacers 125a, 125b, 125c, . . . stacked together in stages.
The stationary blade spacers 125 are ring-shaped members, which are formed of a metal, such as aluminum, iron, stainless steel, or copper, or an alloy containing such metal as a component.
In the outer periphery of the stationary blade spacers 125, the outer cylinder 127 is secured in position with a small gap therebetween. At the bottom of the outer cylinder 127, there is arranged a base portion 129, and a threaded spacer 131 is arranged between the lowermost one of the stationary blade spacers 125 and the base portion 129. In the portion of the base portion 129 below the threaded spacer 131, there is formed a discharge outlet 133 which communicates with the outside.
The threaded spacer 131 is a cylindrical member formed of a metal, such as aluminum, copper, stainless steel, or iron, or an alloy containing such metal as a component, and has a plurality of spiral thread grooves 131a in its inner peripheral surface.
The spiral direction of the thread grooves 131a is determined such that when the molecules of the exhaust gas move in the rotating direction of the rotor 103, these molecules are transferred toward the discharge outlet 133.
Connected to the lowermost one of the rotor blades 102a, 102b, 102c, . . . of the rotor 103 is a rotor blade 102d, which extends vertically downwards. The outer peripheral surface of the rotor blade 102d sticks out toward the inner peripheral surface of the threaded spacer 131 in a cylindrical shape, and is in close proximity to the inner peripheral surface of the threaded spacer 131 with a predetermined gap therebetween.
The base portion 129 is a disc-like member constituting the base of the turbo molecular pump main body 100, and is generally formed of a metal, such as iron, aluminum, or stainless steel.
The base portion 129 physically retains the turbo molecular pump main body 100, and also functions as a heat conduction passage. Thus, the base portion 129 is preferably formed of a metal that is rigid and of high heat conductivity, such as iron, aluminum, or copper.
In the above-described construction, when the rotor blades 102 are driven and rotated by the motor 121 together with the rotor shaft 113, an exhaust gas from a chamber is sucked in through the intake hole 101 by the action of the rotor blades 102 and the stationary blades 123.
The exhaust gas sucked in through the intake hole 101 passes between the rotor blades 102 and the stationary blades 123, and is transferred to the base portion 129. At this point, the temperature of the rotor blades 102 is raised by frictional heat generated as the exhaust gas comes into contact with the rotor blades 102 and by heat generated and conducted from the motor 121. Such heat is transferred to the stationary blades 123 through radiation or through conduction of gas molecules of exhaust gas or the like.
The stationary blade spacers 125 are joined to one another on the outer periphery and send, to the outside, heat which the stationary blades 123 receive from the rotor blades 102 as well as frictional heat generated upon contact between exhaust gas and the stationary blades 123.
The exhaust gas transferred to the base portion 129 is sent to the discharge outlet 133 while being guided by the thread grooves 131a of the threaded spacer 131.
In the description above, the threaded spacer 131 is placed on the outer periphery of the rotor blade 102d and the inner peripheral surface of the threaded spacer 131 is scored with the thread grooves 131a. This may be reversed and the outer peripheral surface of the rotor blade 102d may be scored with thread grooves, whereas a spacer of which inner peripheral surface forms a cylindrical shape surrounds the rotor blade 102d. 
Further, in order to prevent the exhaust gas sucked in through the intake hole 101 from entering the electrical portion composed of the motor 121, the lower radial electromagnet 105, the lower radial sensor 108, the upper radial electromagnet 104, the upper radial sensor 107, etc., the electrical portion is covered with a stator column 122, and the interior of this electrical portion is maintained at a predetermined pressure with a purge gas.
For this purpose, the base portion 129 is equipped with piping (not shown in the drawing), and the purge gas is introduced through the piping. The purge gas introduced is passed through the gap between a protective bearing 120 and the rotor shaft 113, the gap between the rotor and stator of the motor 121, and the gap between the stator column 122 and the rotor blades 102 before it is transmitted to the discharge outlet 133.
The turbo molecular pump main body 100 requires control based on individually adjusted specific parameters (e.g., identification of the model and characteristics corresponding to the model). To store the control parameters, the turbo molecular pump main body 100 contains an electronic circuit portion 141 in its main body. The electronic circuit portion 141 is composed of a semiconductor memory, such as EEP-ROM, electronic parts, such as semiconductor devices for access to the semiconductor memory, a substrate 143 for mounting these components thereto, etc.
This electronic circuit portion 141 is accommodated under an RPM sensor (not shown in the drawing) near the center of the base portion 129 constituting the lower portion of the turbo molecular pump main body 100, and is closed by a hermetic bottom cover 145.
In some cases, a process gas is introduced to a chamber with its temperature raised in order to enhance the reactivity. Such process gas is cooled upon discharge and, reaching a certain temperature, could change into a solid to precipitate in the exhaust system. This type of process gas, one that becomes solid when cooled, adheres to the interior of the turbo molecular pump main body 100 and builds up.
For instance, a vapor pressure curve shows that SiCl4 used as a process gas for an Al etching device precipitates at low vacuum (760 torr to 10−2 torr) and low temperature (about 20° C.) to produce a solid product (e.g., AlCl3), which adheres and builds up in the turbo molecular pump main body 100. As the precipitate of the process gas builds up in the turbo molecular pump main body 100, the pump flow path is clogged with the deposit, thereby lowering the performance of the turbo molecular pump main body 100. The solid product tends to coagulate and adhere in the area near the discharge outlet where the temperature is low, in particular, around the rotor blades 102 and the threaded spacer 131.
A conventional measure taken to solve this problem is to wind a heater (not shown in the drawing) and a ring-like water-cooled tube 149 around the outer periphery of the base portion 129 or other portion while burying a temperature sensor (not shown in the drawing) (e.g., thermistor) in, for example, the base portion 129, so that the temperature of the base portion 129 is kept high at a set temperature by controlling the heating effect of the heater and the cooling effect of the water-cooled tube 149 based on a signal from the temperature sensor (temperature management system, hereinafter abbreviated as TMS).
Given next is a detailed description of the magnet bearing control circuit 201 for exciting and controlling the upper radial electromagnets 104, the lower radial electromagnets 105, and the axial electromagnets 106A and 106B of the turbo molecular pump main body 100 and the control device 200 structured as above.
A structural diagram of the magnetic bearing control circuit and a control circuit is shown in FIG. 15.
In FIG. 15, the control device 200 has the magnetic bearing control circuit 201 provided for the respective electromagnets including the upper radial electromagnets 104 and the lower radial electromagnets 105. Accordingly, in the case of a 5-axis control magnetic bearing, there are ten of the same magnetic bearing control circuits (each of which is denoted by 201 and only some of them are shown in the drawing) in the control device 200.
The magnetic bearing control circuit 201 has a PID control circuit 203 to which a displacement signal sent from the upper radial sensor 107 or other sensors is inputted. The PID control circuit 203 performs PID control on the displacement signal inputted, and outputs as a current command signal a current value necessary to drive the upper radial electromagnets 104 (the current value is hereinafter referred to as current command value) to a current error computing unit 205.
The current error computing unit 205 calculates an error between the current command signal outputted from the PID control circuit 203 and an electromagnet current detection signal outputted from an amplifier circuit 210, (the error is hereinafter referred to as current error value) which will be described later. The current error computing unit 205 then outputs the obtained current error value as a current error signal to a pulse control circuit 207.
The pulse control circuit 207, along with the amplifier circuit 210, will be described next.
A circuit diagram of the amplifier circuit is shown in FIG. 16.
In FIG. 16, the electromagnet coil 151, which constitutes the upper radial electromagnets 104 or other electromagnets, is connected at one end to a positive electrode 221a of a power source 221 through a transistor 211 and is connected at the other end to a negative electrode 221b of the power source 221 through an electromagnetic current detecting circuit 231 and through a transistor 212.
The transistors 211 and 212 are so-called N type power MOSFETs. The transistor 211 has at one end a drain terminal 211a connected to the positive electrode 221a and has at the other end a source terminal 211b connected to the one end of the electromagnet coil 151. The transistor 212 has at one end a drain terminal 212a connected to the electromagnet current detecting circuit 231 and has at the other end a source terminal 212b connected to the negative electrode 221b. 
On the other hand, a diode 215 provided for current regeneration has a cathode terminal 215a connected to one end of the electromagnet coil 151 and has an anode terminal 215b connected to the negative electrode 221b. Similarly, a diode 216 for current regeneration has a cathode terminal 216a connected to the positive electrode 221a and has an anode terminal 216b connected to the other end of the electromagnet coil 151 through the electromagnetic current detecting circuit 231.
The electromagnet current detecting circuit 231 is, for example, a hole sensor serving as a current sensor, and detects the amount of a current flowing in the electromagnet coil 151 (hereinafter referred to as electromagnet current iL) to output the detected current value as an electromagnet current detection signal to the current error computing unit 205.
Also provided between the positive electrode 221a and negative electrode 221b of the power source 221 is a stabilizing capacitor 223 for stabilizing the power source 221.
A node P, which designates the section between the one end of the electromagnet coil 151 and the transistor 211, and a node Q, which designates the section between the other end of the electromagnet coil 151 and the electromagnet current detecting circuit 231, constitute a cable 170 for connecting the control device 200 to the turbo molecular pump main body 100 as shown in FIG. 15, which is because the electromagnet coil 151 is an element of the turbo molecular pump main body 100.
As the magnetic bearing controlling circuit 201 is provided for the respective electromagnets including the upper radial electromagnets 104 and the lower radial electromagnets 105, the amplifier circuit 210 structured as above is provided for each of those electromagnets, meaning that there are identical amplifier circuits (each of which is denoted by 210) for the lower radial electromagnets 105 and for the axial electromagnets 106A and 106B in addition to the amplifier circuit 210 for the upper radial electromagnets 104.
The pulse control circuit 207 determines the pulse width (pulse width time Tp1, Tp2) of pulses to be generated within a control cycle Ts, which is one cycle by PWM control, based on a current error signal outputted from the current error computing unit 205 in order to increase or decrease the electromagnet current iL. At this point, the pulse control circuit 207 receives a carrier wave having a given cycle (for example, 25 kHz) and the control cycle Ts is determined in accordance with the cycle of the carrier wave. The pulse control circuit 207 thus outputs a signal having the pulse width time Tp1 or Tp2 (hereinafter referred to as gate drive signal) within the control cycle Ts to gate terminals of the transistors 211 and 212, to thereby switch on or off the transistors 211 and 212.
In this structure, when a current command value outputted from the PID control circuit 203 is larger than a current detection value detected at the electromagnet current detecting circuit 231, in other words, when the electromagnet current iL is to be increased, the transistors 211 and 212 are both kept turned on for a time period corresponding to the pulse width time Tp1 within the control cycle Ts and are both kept turned off during a time period corresponding to the pulse width time Tp2 (=Ts−Tp1) as shown in FIG. 17.
While the transistors 211 and 212 are both kept turned on, the electromagnet current iL flowing from the positive electrode 221a to the negative electrode 221b through the transistor 211, the electromagnet coil 151, and the transistor 212 is supplied to the electromagnet coil 151 (the electromagnet current iL is increased during this period). On the other hand, while the transistors 211 and 212 are both kept turned off, the electromagnet current iL regenerated from the negative electrode 221b to the positive electrode 221a through the diode 215, the electromagnet coil 151, and the diode 216 is supplied to the electromagnet coil 151 (the electromagnet current iL during this period is smaller than when the transistors 211 and 212 are both turned on).
Therefore, the electromagnet current iL within one control cycle Ts is (ultimately) increased by setting the pulse width time Tp1 longer than the pulse width time Tp2.
This is all reversed in the case where a current command value outputted from the PID control circuit 203 is smaller than a current detection value detected at the electromagnet current detecting circuit 231, in other words, when the electromagnet current iL is to be decreased. The electromagnet current iL within one control cycle Ts is decreased by setting the pulse width time Tp2 longer than the pulse width time Tp1.
By turning one of the transistors 211 and 212 of the amplifier circuit 210 on, a flywheel current is maintained in the amplifier circuit 210 (not shown in the drawing) as disclosed in JP 3176584 B.
Maintaining a flywheel current in the amplifier circuit 210 makes it possible to reduce the hysteresis loss in the amplifier circuit 210 and lower the total current consumption of the circuit. In addition, with the transistors 211 and 212 controlled in this manner, high-frequency noise of harmonics or the like is reduced in the turbo molecular pump main body 100.
As has already been described, the magnetic bearing control circuit 201 is provided for the respective electromagnets including the upper radial electromagnets 104 and the lower radial electromagnets 105 and accordingly the control device 200 has ten identical amplifier circuits (each denoted by 210). Taking into account this and the fact that, in each amplifier circuit 210, two transistors 211 and 212 and two diodes 215 and 216 are necessary for every electromagnet coil 151, the control device 200 needs twenty transistors and twenty diodes in total to drive, through excitation, every electromagnet.
Meanwhile, the transistors 211 and 212 and the diodes 215 and 216 have to be large in element size (channel width for the transistors and junction area for the diodes) in order to supply a large current to the electromagnet coil 151.
It is therefore difficult to downsize the amplifier circuit 210 while ensuring enough current supply to the electromagnet coil 151 and, as a result, reducing the control device 200 in size is made difficult.
The control device 200 therefore takes up much space when installing the turbo molecular pump main body 100 and the control device 200 in a clean room or the like, and it could lead to an increase in cost for the installation.
In addition, as described above, the nodes P and Q that connect the amplifier circuit 210 and the electromagnet coil 151 to each other constitute the cable 170 laid between the control device 200 and the turbo molecular pump main body 100 for interconnection. With the control device 200 having ten amplifier circuits (210), there are twenty wires in total as the nodes P and Q in the cable 170.
Consequently, the cable 170 cannot be reduced in number of wires, thereby making it difficult to lower the cost of the cable 170.
Furthermore, the wires serving as the nodes P and Q have to be large in diameter in order to send a large current to the electromagnet coil 151.
This makes it difficult to reduce the cable 170 in diameter and, accordingly, downsizing of a connector (not shown in the drawing) serving as an entrance and an exit of the turbo molecular pump main body 100 for the cable 170 is also made difficult. The connector on the side of the turbo molecular pump main body 100 bears a special task to enable the cable 170 to serve as an input/output cable while maintaining the vacuum state in the turbo molecular pump main body 100. Therefore, a difficulty in reducing the diameter of the connector could directly lead to a rise in total manufacture cost of the turbo molecular pump.